FI73016C - SAETT VID TILLVERKNING AV MEKANISK CELLULOSAMASSA. - Google Patents

Description

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The way to make mechanical cellulose pulp

Bleached mechanical pulps of the abrasive, milling and hot milling (so-called TMP) type and chemically modified hot milling (so-called CTMP), ie pulps with yields of more than 90%, have been used in the most demanding hygiene products such as shredded and tissue paper. Achieving good absorption properties requires a low resin content. Therefore, for the best qualities, the DKM extract content of the pulp must be less than 0.2%.

This is achieved relatively easily in an open system where the pulp is washed in one way or another with significant amounts of water between each process step and where the pulp is bleached with peroxide. If the water supply is sufficient, a common circulating water system can even be used, which is quite common for practical reasons.

The resin does not dissolve in the washing liquid, but disperses as colloidal or coarser particles in the liquid, making alkaline conditions more desirable. The possibility of reusing circulating water to a greater extent has been seriously suspected except at low resin concentrations. It has therefore been industrial practice for the products in question not to close water systems more than that 3 as a whole consumes at least 15-20 m of water ptm ^. The small resin length apparently could not be maintained at a lower water consumption level.

However, lower water consumption is important, primarily for wastewater treatment methods, but also for heat recovery from the process. Interesting effects are obtained if water consumption is reduced to 10 m ptm _, ..

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Counter-current washing is known and is almost always used to recover concentrated waste liquors from various cooking methods. In this case, it is mainly a question of a completely dissolved substance which does not really cause particular difficulties in washing. However, in sulphate pulps made from high-resin wood species, the colloidal resin may, under certain conditions, desalt and make washing more difficult. However, this problem is generally solved by washing the remaining resin away afterwards in a suitably arranged multi-stage bleaching with proper washes with a fairly high water consumption. It has not been considered natural to directly link mechanical pulping and similar manufacturing techniques and countercurrent washing methods to recover waste liquors from different cooking processes, as it has been difficult to predict how a fairly coarsely dispersed resin will behave in a mechanical pulp filtration process. through.

However, it has now been shown that such an interconnection is possible provided that the conditions are adapted to the special requirements of the resin in the case of mechanical pulp.

Mechanical pulps, TMP or CTMP, are generally defibered in a mild acidic or neutral environment, which does not result in a stable resin dispersion, although mechanical processing in connection with the release of the fibers may be ideal for this purpose. For example, if peroxide bleaching is subsequently followed in an alkaline environment, the alkaline environment will facilitate dispersion of the resin.

Washing between these two process steps can be very interesting because a lower level of resin content in the peroxide solution per se reduces the risk of coagulation of the resin particles and thus improves the stability of the 3 7301 6 resin removal. However, if at the same time the used bleaching solution is recycled in the bleaching step, which is advantageous both in terms of Reducing water consumption and more efficient utilization of bleaching chemicals, the resin appears to reach a desalting concentration unexpectedly quickly. In this case, it is important that the resin from which the salt has thus been removed is washed fairly quickly with a washing water whose ionic strength gradually decreases up to pure water. Washing must be carried out mainly by displacement and must be carried out to a high degree of displacement of the solute. To minimize water consumption, the pre-peroxide wash must be performed in series with the post-peroxide wash. The washing liquid is led as a uniform countercurrent.

Quite unexpectedly, it has been found that such a method can produce pulps with acceptably low resin contents even with very low water consumption.

The unbleached CTMP pulp was prepared in a pilot plant by impregnating spruce chips containing about 1.2% resin with a sodium sulfite solution, whereby the chips absorb about 20 kg of Na 2 SO 4 / tn dry-thought chips. The chips were preheated with steam to 110 ° C and then ground in a chip mill. The mass concentration after the chip grinder was measured to be about 30% by weight. The pulp was diluted to a concentration of about 4%, after which it was dewatered in a press to a concentration of about 30%. The resin content of the pulp after the press was analyzed.

Several experiments were performed, in which the mass was diluted with water or a mixture of water and circulating water after the grinder but before the press. By using less and less fresh water and more and more circulating water to dilute the pulp, the degree of closure of the system could be increased.

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Analyzes of the resin content of the pulp gave the relationships shown in Figure 1 between the resin content of the pulp and the amount of fresh water fed to system 3 (m fresh water / tn pulp).

By drawing material balances for the resin in each experiment, it was possible to calculate how much resin was dispersed in the circulating water after grinding and how much can be removed by dewatering and washing, and correspondingly how much resin cannot be removed in this way. The following results have been obtained.

Amount of fresh water added Amount of resin that can be removed by washing or removal of water m / tn mass,% 5 0.25 10 0.45 20 0.57 30 0.61

The results of Figure 1 illustrate the findings underlying the present invention. In the production of unbleached CTMP pulps, a higher barrier means that less and less wood is dissolved or dispersed from the resin and the resin content of the pulps increases. Similar studies in the manufacture of hot mills have yielded similar results. The results can be interpreted as giving a higher desalting of the resin released or dispersed in the defibering when the barrier is added. In the production of unbleached mechanical pulps with applications where the pulps must have a low resin content, it has not been possible to close the systems sufficiently in the past.

The pulps obtained from the above experimental plant were then subjected to alkaline peroxide bleaching. Alkaline peroxide bleaching has e.g. the following purposes: it improves the brightness of the pulp it reduces the resin content of the pulp.

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The experiments were performed by diluting unbleached TMP and CTMP pulps after the press with fresh water or circulating water from the bleaching step. The bleaching chemicals were mixed into the pulp suspension at a pulp concentration of about 12%, after which the pulp suspension was passed to a bleaching tower. After bleaching, the pulps were diluted to a pulp concentration of 4% with fresh water or circulating water, after which the pulps were pressed to a pulp concentration of about 35%. The resin content of the pulp samples was determined after the press.

The table below shows a few examples of the pulps used in the bleaching tests. Chemical inputs in the bleaching step are shown.

Pulp Pulp resin- Added pulp Bleaching chemical content fresh water resin charge kg / tn pulp before pair- before concentration

bleach bleach press- H 2 O NaOH Na EDTA

% By weight of herring 0% CateP 1 0.84 7.1 0.49 37 25 40 3.0 CTMP 2 0.81 23.9 0.27 25 18 50 3.8 CM ? 3 0.88 26.6 0.32 42 30 42 3.1 TMP 1.04 26.3 0.48 46 31 43 3.2

Here, too, by drawing up the material and resin balances for the bleaching step, it was possible to calculate how much of the resin was dispersed and thus washable and, correspondingly, how much of the resin was not washable. Em. the masses studied in the table gave the following results.

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Pulp Added Pulp Pulp Dispersed Temperature Fresh Water Resin Resin Mat Bleach Bleach Concentration Resin Stage Stage Oral Bleach Concentration O 2 and Before and After Bleach Bleach Press Bleach

~ V O

rn / tn% mass CIMP 1 5.5 0.49 0.16 0.05 70 CTMP 2 21.1 0.27 0.15 0.13 70 CIMP 3 18.4 0.32 0.15 0.13 60 ΊΜΡ 23.3 0.48 0.19 0.13 85

In bleaching, most of the remaining resin is released. Alkaline peroxide bleaching then converts the resin to such a form that it does not precipitate when the barrier is added, at least if the barrier is kept within reasonable limits. The water consumption in the barrier test performed with CTM mass 1 corresponds to 3 values of approx. 6-10 m / tn mass.

Experimental studies on the production of bleached mechanical pulp have given the following results.

1. In the manufacture of unbleached mechanical pulps, hot mills and chemically modified hot mills, part of the resin dissolves or disperses in wood. The dispersed resin can be removed by washing.

2. When the unbleached pulp circulating water system is closed, an ever smaller part is dispersed. The resin content of the unbleached pulp increases as the circulating water barrier increases.

3. In the bleaching of these pulps, the resin changes to such a form that the bulk of the resin is dispersed. The resin has been modified in bleaching so that it no longer precipitates when the barrier is added.

7 7301 6 4. The amount of non-dispersible resin in the peroxide bleached mechanical pulp is only 0.05-0.15% of the pulp.

These observations allow the production of a bleached mechanical pulp with a very low resin content with low water consumption if the pulp is washed before and after bleaching and if the circulating water is directed countercurrent to the pulp.

An embodiment of the invention is shown in Figure 2, but the invention is of course not limited to this embodiment.

The system comprises a grinding step 1, a dewatering step or a washing step 2, a bleaching step 3, a washing step 4, a dewatering and / or washing step 5 and a drying step 6 in which the bleached pulp is dried.

Chip is fed to grinding step 1, which may be untreated, steamed and / or chemically impregnated. After the grinder 1, the pulp is passed to a washing stage 2, which may be a washing filter or suitably a dewatering or washing press. Before the washing stage, the pulp suspension is diluted with circulating water from the bleaching stage and, if necessary, with circulating water from washing stage 2.

After the washing step 2, the unbleached pulp is fed with bleaching chemicals as well as the dilution water coming from the washing step 4, and the pulp is fed to the bleaching tower 3.

After bleaching, the pulp is diluted with circulating water from washing step 4 and washed in washing step 4. The displacement liquid is circulating water from washing and dewatering step 5. The pulp is dewatered before dryer 6 in dewatering step 5, which may suitably be a dewatering or washing press. The circulating water leaves the system via washing step 2. The washing liquid is added in connection with washing step 5.

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The obtained power can be illuminated as follows. The CTMP chips are ground in step 1, where the mass concentration after the grinder is 30%. The pulp is dewatered to a pulp concentration of 48% in a dewatering press 2. The pulp is then bleached to a pulp concentration of 12%, washed with a washing filter 4 and finally dewatered in a dewatering press 5 to a pulp concentration of 48% before the dryer 6.

Using the automatic model and the obtained test results, the resin balances of the system were calculated at different degrees of closure.

The following correlations were obtained between the amount of circulating water leaving the system, the washing efficiency of the resin released or dispersed in the washing step, and the resin content of the bleached pulp. The non-dispersible resin content of the pulp in the bleaching step is assumed to be 0.13%. The resin content of the pulp after grinding is assumed to be 0.9%.

Leaving circulating water- Bleaching stage Bleached pulp .....; 3 resin content of the resin released, m / tn mass, .... .0 "washing efficiency% by weight 1% 0.77 0.31 3 0.87 0.20 5 0.92 0.16 10 0.95 0.15

The calculation shows that by sealing the system well, bleached mechanical pulp with a low resin content can be produced, provided that the pulp is washed countercurrently.

Peroxide bleaching of mechanical pulp or the like (pulps> 90%) gives a very large amount of residual chemical under normal conditions, which cannot simply be exploited by tightening the bleaching conditions, but which can be utilized to a greater or lesser extent if the residual chemical solution is returned to the unbleached . Performing such a restoration requires better equipment normally used and better monitoring of water systems. Quite successful experiments have been performed with such a recovery, but the efficiency has proven to be low, probably because dilution phenomena and other interfering effects have affected the process in a rather incomprehensible way.

It has now completely unexpectedly been shown that, under carefully designed conditions, a bleaching efficiency of almost 100% can be obtained with these residual chemicals. The condition is that they are returned with only a slight dilution and that the circulating water from the grinding step is allowed to cause only a small amount of dilution before bleaching. This, in turn, requires a precisely balanced circulating water system in which the circulating water of the grinding stage and the circulating water of the bleaching stage are properly separated.

The invention is not limited to the described embodiments, but can be varied within the scope of the invention.

Claims (8)

A method of preparing cellulose pulp in a yield exceeding about 90% to obtain a pulp substantially free from resin, by mechanical defibration of wood material followed by bleaching of the defibrated material, the method being carried out in a closed system corresponding to a the amount of wastewater not exceeding 15 m 2 per tonne of pulp, and that the method is carried out in at least one defibration step (1), at least one wash step (2), in which the pulp is pressed and washed after defibration, at least one bleaching step (3) and at least one wash step ( 4) for the bleached pulp, characterized in that, after the bleaching, the washing step (4) is supplied with a washing liquid, backwater from the washing stage (4) after the bleaching is used partly for dilution of the pulp after the washing step (2) and partly for dilution and washing. of the pulp from the defibration step (1), backwater from the defibration step (1) and from the bleaching step (3), separate from each other is maintained so that backwater from the defibration step (1) does not exceed The pipes of the bleaching stage's backwater system to be removed from the process are withdrawn in the washing step (2) arranged after the defibration step (1) and that the washing power of the process is poured at least at least 70% for eating. 2. A method according to claim 1, characterized in that the bleaching step consists of an alkaline bleaching step, preferably peroxide bleaching. Method according to claim 1, characterized in that the defibration step is a refiner step. 4. A method according to claim 1, characterized in that backwater from defibration step and bleaching step